The actin filament is a key element in the dynamic machinery of motile cells and in the cytoskeleton of non-mobile cells. Different assemblies of actin filaments fulfill different roles, even within the same cell. These assemblies differ both in their function and in their filament organization. We propose experiments to discover the properties of actin which make it a common element in the cytoskeletal structures. We also plan to study how these cytoskeletal structures are constructed. The approach, using combined techniques of electron microscopy and image analysis, is to study the structure of isolated actin filaments, free and in combination with other proteins, actin bundles, and a cytoplasmic actin gel. In particular, we plan: 1) a study of actin, myosin S-1 decorated actin, thin filament, and the actin-containing filaments from the acrosomal bundle in Limulus; 2) a comparative study of the structures of actin-fascin and actin-fimbrin bundles; 3) a study of the morphogenesis of the actin-scruin bundle found in the sperm of Limulus, the horseshoe crab; 4) a study of the structure, organization and morphogenesis of a naturally occurring actin gel, the cuticular plate, found in hair cells in the inner ear. The aim of these studies is to reconstruct the three-dimensional structure of actin; to determine the shapes of myosin S-1, scruin, fimbrin, and fascin; to locate their binding sites on actin; and to determine how their bonding modifies the structure of the actin filament. Next, we aim to determine the bonding rules for actin filaments in the cuticular plate, a naturally occurring actin gel. Finally, we aim to determine how bundles and gels are assembled in vivo by looking at the formation of these structures in development.